1. Field of the Invention
This invention relates to determining the amount of exhaust gas that is added to the intake air going to the cylinders of an internal combustion engine.
2. Prior Art
Exhaust gas recirculation (EGR) is commonly used in vehicles with internal combustion engines to reduce the amount of NO.sub.x produced in the engine cylinders during combustion. Depending on engine operating conditions, a certain amount of exhaust gas is added through an EGR valve to the intake air that goes to the cylinders. The dilution of the intake air charge with exhaust gas results in a lower combustion temperature and thus production of smaller amounts of NO.sub.x. EGR is usually measured as percentage of exhaust gas in the combined air and exhaust intake mixture. The amount of EGR is determined by the degree of opening of the EGR valve and the difference in gas pressure across the valve. Usually, EGR is measured with a position sensor that measures the degree of opening of the EGR valve. This type of measurement is not reliable, however, because a) deposits can partially block the valve and b) changes in back pressure result in changes in the amount of EGR. Another method of determining EGR is based on the measurement of the flow of the exhaust gas added to the intake air. This is done by measuring with two pressure sensors the pressure drop across an orifice through which the recirculated exhaust gas is passed. This method of determining EGR also has problems because the orifice can be partially blocked by deposits. The present invention describes a method for measuring EGR and an EGR sensor which eliminate these problems.